Facebook Authentication for signing into online account

We are embarking on a project to allow our customers to login to their accounts using Facebook to authenticate. We are locally hosted. 

Are there any other non-TNEW users who are doing this? I'd be interested to know  

• what issues, if any, did you run into during the development of Facebook authentication?
• What information do you get from Facebook (demographic, interests) and where do you store it in Tess? 
• Anyone who uses Facebook events to track who is coming or interested, and is that connected to the actual purchase of tickets? (for instance, once you check out, are able to share with friends that you are attending the concert this weekend?)

Thanks in advance to anyone willing to share their experience.